Because weight and high temperature strength are primary considerations in gas turbine engine design, there is a continuing effort to create relatively light weight alloys which have high strength at elevated temperatures. Titanium-based alloy systems are well known in the prior art as having mechanical properties which are suitable for relatively high temperature applications, with a practical upper limit being generally about 1100.degree. F. However, as a result, these titanium-based alloys are typically not practical for many high temperature gas turbine engine applications which require usage at temperatures much higher than 1100.degree. F. Thus, for many of these high temperature gas turbine applications, the use of heavier superalloys that are roughly twice as heavy as titanium-based alloys is necessitated.
The high temperature capability of titanium-based alloys has been gradually increased by the use of titanium intermetallic systems based on the titanium aluminides Ti.sub.3 Al (alpha-2 alloys) and TiAl (gamma alloys). Generally, Ti.sub.3 Al-based alloys typically contain aluminum in amounts between about 23 and about 25 atomic percent, and TiAl-based alloys typically contain aluminum in amounts between about 46 and about 52 atomic percent. These titanium aluminide alloys are generally characterized as being relatively light weight, yet exhibit high strength, creep strength and fatigue resistance at elevated temperatures of up to about 1830.degree. F., according to the ASM Handbook, vol. 2, p. 926 (1990).
However, these binary titanium aluminide alloys have a significant shortcoming in terms of their low ductility and corresponding brittleness and low fracture toughness at room temperature, which makes them difficult to process. In addition, these alloys do not exhibit desired high oxidation resistance due to their tendency to form titanium dioxide (TiO.sub.2 ) rather than aluminum oxide (Al.sub.2 O.sub.3) at high temperatures. For example, the oxidation limit for the gamma TiAl alloys is significantly less than its creep limit of 1830.degree. F. Accordingly, a common objective with the use of titanium aluminide alloys is to achieve a good balance between mechanical properties at both room temperature and elevated temperatures, and environmental characteristics, such as oxidation resistance.
Gamma TiAl alloys, such as Ti-48Al-1V (atomic percent), generally possess temperature capabilities and densities which are superior to that of the Ti.sub.3 Al alpha-2 alloys. As a result, gamma TiAl alloys generally have greater potential as an alloy suitable for the high temperature applications of gas turbine engines. However, the Ti-48Al-1V alloy has been found to be susceptible to a relatively rapid rate of oxidation at temperatures between about 1400.degree. F. and about 1600.degree. F. To solve this shortcoming, it is known to add niobium and/or tantalum to improve the oxidation resistance of the alloy. This oxidation resistance is largely the result of an improvement in the physical and chemical properties of an oxidized layer which forms on the alloy as a protective coating. When the alloy is exposed to an oxidizing environment, the protective coating forms which is essentially a mixture of titanium dioxide and alpha alumina.
In addition, niobium and tantalum are known to improve the strength of the TiAl alloys. However, niobium and tantalum are generally considered to reduce ductility, an adverse condition which already exists in conventional TiAl alloys.
It is also known to add tungsten to improve the oxidation resistance of titanium aluminide alloys. In addition, tungsten additions are also known to significantly improve the creep strength behavior of titanium aluminide alloys. However, as with niobium and tantalum, tungsten is also generally considered to reduce the ductility of an alloy, which would be expected to further exacerbate the low ductility seen in conventional TiAl alloys.
For improving ductility, alloying additions of vanadium, chromium and manganese have been reported to be effective. However, these alloying elements are also known to decrease oxidation resistance of the alloy. Accordingly, the need to achieve a balance between the mechanical properties and the environmental capabilities of gamma titanium aluminides is characterized by offsetting factors, so that this balance has not been realized in the prior art. This balance is further complicated by the desire for an alloy to be extrudable, forgable, rollable and castable, so as to enable the fabrication of various types of components, such as those for gas turbine and automotive engines. Yet it is also desirable for the alloy to be responsive to heat treatments, so as to permit tailored microstructures and mechanical properties for specific applications.
Thus, it would be desirable to provide a titanium aluminide alloy which exhibits both sufficiently high strength, creep resistance and oxidation resistance at elevated temperatures, while also being sufficiently ductile and fracture tough at room temperature so as to enable the alloy to be more readily processed, and thereby more readily permit the fabrication of relatively light weight components which can be tailored for use in high temperature environments, such as found within gas turbine as well as automotive engines.